1. Field of the Invention
The present invention relates to coatable urea-aldehyde binder precursor compositions having low free aldehyde content which are catalyzed to cured binders by a cocatalyst. The catalyst is described as a "cocatalyst" because it has two components: an ammonium salt (such as ammonium chloride, ammonium nitrate, ammonium thiocyanate, and the like) and a Lewis acid (such as aluminum chloride, ferric chloride, and the like). The cocatalyst is especially useful in the production of coated abrasive articles.
2. Description of Related Art
The use of acid/base reactions to control the addition and condensation reactions of urea-formaldehyde (UF) dates back to the 1918 work of Hanns John. (This discussion uses urea-formaldehyde merely as the preferred resin and for purposes of discussion.) It is generally accepted that a nucleophilic component is necessary for an amino-carbonyl condensation via reactions 1-3 (all aqueous): ##STR1##
Although the addition reaction (reaction 2) is both acid and base catalyzed, the condensation reaction (reaction 4) is exclusively acid catalyzed: EQU NH.sub.2 CONHCH.sub.2 OH+H.sub.2 NCONH.sub.2 .rarw.H.sup.+ .fwdarw.NH.sub.2 CONHCH.sub.2 NHCONH.sub.2 +H.sub.2 O. 4)
The nucleophilic component necessary for amino-carbonyl condensations can be provided by any of a variety of proton donors. The most common classes are mineral acids, OH--acidic compounds, acidic SH, NH and CH moieties, and some olefins.
UF was first patented for use as an adhesive for coated abrasives by Minnesota Mining and Manufacturing Company ("3M") in the mid 1930's (Great Britain Patent No. 419,812). Since that time a number of different coated abrasive products have been made with acid catalyzed UF resins. Today, the two most common catalysts used with UF resins are aluminum chloride (AlCl.sub.3) and ammonium chloride (NH.sub.4 Cl).
Coated abrasives typically comprise a backing such as paper, cloth, and the like, which has adhered thereto (with a binder) a plurality of abrasive particles. One typical binder used in coated abrasives is a condensation copolymerization reaction product of an aldehyde with urea and/or urea derivatives. Urea-aldehyde binders possess uniformity of physical properties whereby any desired, predetermined degree of thickness and color binder may be obtained, while still maintaining a desirable degree of flexibility of the coated abrasive. Urea-aldehyde binders are also resistant to a wide range of liquids used in sanding operations, such as water, organic solvents or inorganic materials, such as acids or alkalis.
Although urea-aldehyde resins have enjoyed great success in coated abrasives, the need to reduce the use of solvents and unreacted reactants which contribute to release of volatile organic hydrocarbons (VOC) in the process of making coated abrasives, and the need to increase the quality of the abrasives while maintaining or increasing their level of performance is challenging the industry.
Meanwhile, the appearance to the user of the coated abrasive is important. It has been interestingly found that, when attempting to increase the abrading performance of coated abrasives made using urea-aldehyde resins when aluminum chloride is used alone as the catalyst, a higher temperature than normal must be used to cure the urea-aldehyde resin, which in turn leads to curling of edges of the coated abrasive. (The use of aluminum chloride as a catalyst for urea-formaldehyde resins in the making of coated abrasive articles is known.) Therefore, it would be advantageous if the abrading performance of coated abrasives made using urea-aldehyde resins could be increased without sacrificing the appearance or increasing the waste of coated abrasive.
When the AlCl.sub.3 catalyst is used alone, the gel time, pot life and peak exotherm temperatures are all dependent on the concentration of the AlCl.sub.3, Thus, the performance of the coated abrasive is dependent upon the concentration of the AlCl.sub.3, and the cure conditions (time and temperature).
In order to achieve a good performing product using factory cure conditions (i.e temperature ranging from about 65.degree. C. to about 95.degree. C.), the concentration of AlCl.sub.3 should be near 1 weight percent, based on weight of binder precursor. The drawback with a 1 weight percent concentration of AlCl.sub.3 is that the pot-life may be too short for batch operations typically used in the factory with urea-aldehyde resins having low (about 0.1 to about 1.0 weight percent) free aldehyde content, based on total weight of aldehyde.
When NH.sub.4 Cl is used alone as the catalyst, the gel time, pot life and peak exotherm temperatures are all independent of the NH.sub.4 Cl concentration, affording an advantage over the use of a Lewis acid catalyst. However, the activity (ability of the catalyst to catalyze the reaction) of the NH.sub.4 Cl was dependent on the free formaldehyde concentration in the binder precursor composition due to the following reaction: EQU 6CH.sub.2 (OH).sub.2 +4NH.sub.4 Cl.fwdarw.(CH.sub.2).sub.6 N.sub.4 +4HCl+12H.sub.2 O. 5)
With low free aldehyde resins, such as that known under the trade designation AL3029R from Borden Chemical, the NH.sub.4 Cl does not activate the condensation reaction (4) very readily until the temperature of the reaction is increased above that normally used. However, as mentioned above, increased temperature tends to curl the edges of the coated abrasive and does not render performance improvements. The performance of the coated abrasive is independent of the NH.sub.4 Cl concentration. Thus, the drawbacks of this system are the long gel times, and only moderate performance levels are obtained with typical factory cure conditions.
No art is known to the inventors that describes the use of a cocatalyst comprising an ammonium salt and a Lewis acid which is useful in making of coated abrasive articles or any benefit which would be derived therefrom.
Therefore, it would be an advance in the art to provide a binder precursor composition (preferably a solution or dispersion) which includes a urea-aldehyde resin and cocatalyst system and coated abrasives which meet these needs. It is the primary object of the present invention to provide such compositions which will, when cured, provide a coated abrasive binder having uniformity of physical properties as is previously known, but which also allow higher production runs of coated abrasives without curling of the edges of the coated abrasive web and increased abrasion performance.
The above-mentioned Great Britain Patent No. 419,812 gives a good account of the advantages of urea-aldehyde binders in the manufacture of flexible abrasive articles such as coated abrasives. The patentee notes that it is desirable on some occasions that the urea-aldehyde solution gel quickly after application. To achieve this there may be added a water soluble non-alkaline salt having a high degree of electrolytic dissociation, such as ammonium salts of both strong and weak acids. However, there is no disclosure of the use of a Lewis acid in conjunction with an ammonium salt as a cocatalyst system for urea-aldehyde binders.
U.S. Pat. No. 2,518,388 (Simons) describes the preparation of improved urea-formaldehyde adhesive compositions. A hardening catalyst is added to the urea-formaldehyde along with a compound containing a furan ring structure, such as furfuryl alcohol or furfural. A catalyst is made up of three parts by weight ammonium chloride and ten parts by weight water or two parts ammonium chloride, one part ferric chloride and seven parts water. The former catalyst is stated to give a longer working life while the latter catalyst system is said to give a faster cure but a shorter working life of the liquid adhesive. There is no discussion of the use of such a catalyst system with urea-aldehyde resins in a coated abrasive product nor the benefits which might be derived from the use of such a catalyst in the production of abrasive products.
U.S. Pat. No. 3,933,274 (Emmons et. al.) describes a latent acid catalyst which catalyzes the crosslinking of cellulose molecules with formaldehyde in cellulosic fabrics. The crosslinking reaction is conducted under acidic conditions using a latent acid catalyst, which ideally only becomes strongly acidic on exposure to high temperatures so that it does not cause premature crosslinking when the cellulosic fabric is impregnated. Latent acid catalysts are described as usually being inorganic salts, for example aluminum chloride, magnesium sulfate and perfluorate, ammonium chloride, sulfate, nitrate and tartrate. There is no disclosure of the use of the combination of an ammonium salt and a Lewis acid nor the use of such a combination in a binder precursor composition used to form an abrasive product.
U.S. Pat. No. 4,761,441 (Woodson) describes acid-curable resin compositions comprising a mixture of a furan resin and a minor amount of an acid-curable epoxy resin. The acid-curable resin compositions may contain acid catalysts, acid catalysts being described as generally well-known and being either inorganic acids, organic acids, or salts thereof, examples of inorganic acids including ferric chloride, and examples of inorganic salts which are normally used include ammonium chloride, ammonium sulfate, ammonium nitrate, aluminum chloride, and the like. The patent also notes that furan resins have been used as binders in, among other things, grinding wheels and coated abrasives. The patentee notes that mixtures of acids and acid salts can be utilized as the acid catalyst for curing the acid-curable resins; however, there is no suggestion of a mixture of an ammonium salt with a Lewis acid as a cocatalyst for urea-aldehyde resins.
U.S. Pat. No. 4,855,354 (Mohler et. al.) describes a curable aldehyde resin and starch composition useful as a saturant and laminating adhesive for paperboard products. The aldehyde resin possesses a low level of free-formaldehyde, while the adhesive composition is described by Mohler et. al. as curable upon the addition of or exposure to a sufficient amount of curing agent such as aluminum chloride. However, there is no suggestion of the use of a combination of ammonium salt with a Lewis acid as a cocatalyst system, nor the use of such a system for making an abrasive product.
Russian Author's Certificate No. 738745 (published Jun. 5, 1980) describes a heat hardening mold mixture for production of casting rods which comprises a urea-formaldehyde binder, a phenol-formaldehyde binder, a "complex acidic catalyst" and other ingredients The complex acidic catalyst is described as a solution of ammonium chloride, aluminum chloride and technical urea in water. There is no suggestion of the use of a mixture of ammonium chloride/aluminum chloride, or any other cocatalyst system in a coatable urea-aldehyde composition, such as would be useful in the making of an abrasive product.
Coated and nonwoven abrasive articles are described generally in U.S. Pat. Nos. 4,903,440 and 2,958,593, respectively. A technical brochure from Borden Chemical entitled "Durite Abrasive Binder AL8405" describes the urea-formaldehyde resins catalyzed solely by aluminum chloride. The brochure also mentions the use of ammonium chloride, but not as part of a cocatalyst system. See also U.S. Pat. No. 1,355,834, and Meyer, B. "Urea-Formaldehyde Resins", Addison-Wesley Publishing, Boston, (1979). The use of organic amine salts as such as the dichloride salt of hexamethylene diamine as flexibilizers in urea-formaldehyde binder precursor compositions is described by Myers et al., J. of Applied Polymer Science, Vol. 42, p. 2997 (1991).